Gas is becoming more and more attractive fuel for ships' and other marine vessels' prime movers and auxiliary engines. Particularly but not exclusively natural gas (NG) is feasible due to its availability. Natural gas is a gaseous mixture in the ambient circumstances consisting primarily of methane and small amounts of ethane, propane, butane and nitrogen. It has high hydrogen content relative to coal, so when combusted it provides inter alia low amount of emissions, very clean burning process and it is basically free of contaminants. Particularly in cruise vessels, ferries and so called ro-pax vessels, where passengers are on board, the absence of soot emissions and visible smoke in the exhaust gases of ship's engines is very important feature facilitated by using NG as fuel for the engines but also for vessels carrying goods and bulk materials. Usually natural gas is stored as liquefied natural gas (LNG) at temperature of about −162° C., thus the storage cause problems due to the requirements to provide effective insulation. Additional problem is caused if the LNG is stored at high pressure, about at 5-6 bar but also more if needed, which is typically the level that a gas operated piston engine requires.
EP2032428 B1 discloses a fuel system for gas driven piston engine in a marine vessel, which gas is stored at least one fuel storage tank in the vessel as liquefied gas. A separate fuel feed tank in which the gas is in liquid phase and at elevated pressure and separate fuel storage tank or tanks in which the gas is also in liquid phase. Since the liquefied gas is at low temperature the tanks are heat insulated.
Generally, in sake of increased redundancy the possibility of changing the used fuel in the vessel is a desired feature. Liquefied natural gas tanks are not suitable for store or contain any other fuels than gaseous fuels like the liquefied natural gas. For example, because of the double wall vacuum insulation an interior of a common LNG tank cannot be accessed and therefore cannot be cleaned inside.
An object of the invention is to provide a fuel tank arrangement in a marine vessel and a method of operating a tank container wherein different types of fuels are used in a same tank container and thus not replacing the tank container, and the arrangement in which the performance is considerably improved compared to the prior art solutions.